(c) In the neutral flame the temperature at the inner cone tip is
approximately 5,850F, while at the end of the outer sheath or envelope the
temperature drops to approximately 2,300F. This variation within the flame
permits some temperature control when making a weld. The position of the flame to
the molten puddle can be changed and the heat controlled in this manner.
(2) Oxidizing flame.
(a) The oxidizing flame is produced when slightly more than one volume
of oxygen is mixed with one volume of acetylene. The volume ratio is 1.7 to 1.15.
To obtain this type of flame the torch should first be adjusted to give a neutral
flame. The flow of oxygen is then increased until the inner cone is shortened to
about one-tenth of its original length. When the flame is properly adjusted the
inner cone is pointed and slightly purple. An oxidizing flame can also be
recognized by its distinct hissing sound. The temperature of this flame is
approximately 6,300F.
(b) When applied to steel an oxidizing flame causes the molten metal to
foam and give off sparks. This indicates that the excess oxygen is combining with
the steel and burning it. An oxidizing flame should not be used for welding steel
because the deposited metal will be porous, oxidized, and brittle. This flame will
ruin most metals and should be avoided, except as noted in (c) below.
(c) A slightly oxidizing flame is used in torch brazing of steel and
cast iron. A stronger oxidizing flame is-used in the welding of brass or bronze.
(d) In most cases the amount of excess oxygen used in this flame must be
determined by observing the action of the flame on the molten metal.
(3) Reducing or carburizing flame.
(a) The reducing or carburizing flame is obtained when slightly less
than one volume of oxygen is mixed with one volume of acetylene. The volume ratio
is 0.85 to 0.95. This flame is obtained by first adjusting to neutral and then
slowly opening the acetylene valve until an acetylene streamer or "feather" is at
the end of the inner cone. The length of this excess streamer indicates the degree
of flame carburization. For most welding operations this streamer should be
approximately twice the length of the inner cone.
(b) The reducing or carburizing flame can always be recognized by the
presence of three distinct flame zones. There is a clearly defined bluish-white
inner cone, a white intermediate cone indicating the amount of excess acetylene,
and a light-blue outer flame envelope. This type of flame burns with a coarse
rushing sound and has a temperature of approximately 5,700F, at the inner cone
tips.
(c) When a strongly carburizing flame is used for welding the metal
boils and is not clear. The steel in absorbing carbon from the flame gives off
heat which causes the metal to boil. When cold the weld has the properties of high
carbon steel, being brittle and subject to cracking.
(d) A slight feather flame of acetylene is sometimes used for backhand
welding. A carburizing flame is advantageous for welding high carbon steel, for
hard facing operations, and for welding such nonferrous alloys as nickel and monel.
When used in silver soldering operations only, the intermediate and outer flame
cones are used. They impart a low temperature soaking heat to the parts being
soldered.
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